I would first, manually delete the
C:\windows\temp directory and the
C:\Documents and Settings\%UserProfile%\Local Settings\Temp directory. Hint: With XP, sort them by date, and delete everything older than today, then try to delete the rest. The reason to delete them is that your scans will be faster, and often malware hides there, and is easier to see with all the other junk removed. You can safely delete the files in those directories. Take note of what is left. If it is a wild random file name that won't delete, that is probably your problem, or one of them.
Next Install Malwarebytes, Superantispyware, and Hitman Pro that you get from download.com. Boot into Safe mode with Networking and run all three scans. Then run HijackThis from the same download.com. Look at the entries labeled BHO that don't have a company name associated with them. They are probably bad, but you can post your HijackThis log on many sites and have people help you with it specifically. BHO's are designed to run with your browser, but if malware, can infect you every time you open that browser.
Then after all that, go back and look at your temp directories again, and see if you can delete the bad files. If not, download Unlocker, again from download.com, and unlock the bad file and delete it. I did not want to do this earlier since depending on what it is attached to, it could crash the system.
Lastly, if none of this works, you would probably want to try Combofix. It is a bit of a last resort because if you have certain types of viruses in the boot sector, it can clean them, but render the computer unbootable, at least temporarily. Frankly, this does not happen too often.